Chevron-type liquid crystal device having effective display and pattern display regions

ABSTRACT

A liquid crystal display panel includes a pair of substrates having thereon scanning electrodes and data electrodes so as to form an electrode matrix, and a chiral smectic liquid crystal disposed between the scanning electrodes and data electrodes to form a plurality of smectic layers each including a plurality of liquid crystal molecules. The scanning electrodes and data electrodes are driven to define an effective display region and a frame-like fixed or semi-fixed pattern display region disposed outside the effective display region and extending along at least one side of the display panel. The smectic layers of the chiral smectic liquid crystal are aligned to extend in a direction which is non-parallel with the extension of the frame-like fixed or semi-fixed pattern display region so as to obviate the occurrence of cell thickness irregularity and liquid crystal voids.

This application is a continuation of application Ser. No. 08/101,958,filed Aug. 4, 1993, now abandoned, which is a continuation ofapplication Ser. No. 07/918,007, filed Jul. 24, 1992, now abandoned.

FIELD OF THE INVENTION AND RELATED ART

The present invention relates to a liquid crystal device for use in aliquid crystal display device, a liquid crystal shutter, etc., and moreparticularly to a ferroelectric liquid crystal device having improveddurability.

There has been known a liquid crystal device using a ferroelectricliquid crystal (hereinafter sometimes abbreviated as "FLC") whichincludes a cell structure formed by disposing a pair of glass plateseach provided with transparent electrodes and an aligning treatment ontheir inner sides opposite to each other with a cell gap on the order of1 to 3 μm, and a ferroelectric liquid crystal filling the cell gap, asdisclosed in U.S. Pat. No. 4,932,758.

Such a ferroelectric liquid crystal device is characterized in that theferroelectric liquid crystal has a spontaneous polarization so that acoupling force of an external electric field and the spontaneouspolarization can be utilized for switching and that the longitudinalaxis direction of an FLC molecule corresponds one-to-one to thepolarization direction of the spontaneous polarization so that theswitching is effected depending on the polarity of the external electricfield.

A ferroelectric liquid crystal generally comprises a chiral smecticliquid crystal in SmC* or SmH* phase so that the longitudinal liquidcrystal molecular axis is helically aligned but, if the liquid crystalis disposed in a cell structure having a cell gap on the order of 1-3 μmas described above, the helix of the longitudinal liquid crystalmolecular axis is released.

In an actual drive of such a ferroelectric liquid crystal cell, a simpleelectrode matrix as shown in FIG. 2 has been used. It has been foundthat such a ferroelectric liquid crystal cell involves a problemregarding durability as will be described hereinbelow.

It has been known that a liquid crystal molecule moves or fluctuates tosome extent along a helical cone inherent to a chiral smectic liquidcrystal in response to even a non-selection or non-writing signal duringmatrix drive. This is clear from an optical response of a pixelreceiving a non-selection signal including a change in light quantitysynchronized with applied pulses. In an FLC display device using a splayalignment state (i.e., an alignment state wherein longitudinal molecularaxes causes a large angular twist between a pair of substrates), such amolecular fluctuation is not so problematic except for a slight decreasein contrast, as the display state can be maintained if the stableposition is not changed (i.e., not switched).

In a ferroelectric liquid crystal cell utilizing a uniform alignmentstate accompanied with little angular twist of longitudinal molecularaxes between the substrates as shown in U.S. Pat. No. 4,932,758, etc.,however, there has been found a phenomenon that liquid crystal moleculesmove translationally along the liquid crystal layer extension inresponse to applied voltages (e.g., non-selection signals). A remarkableexample of the phenomenon is explained with reference to FIGS. 4A-4D,wherein FIG. 4A illustrates a state before voltage application and FIG.4B illustrates a state after voltage application, respectively, of aliquid crystal device 1. The liquid crystal device 1 includes a pair ofsubstrates 11 provided with polyimide films subjected to rubbing as analigning treatment in upward parallel directions commonly represented byan arrow 42. As a result of the treatment, smectic layers 41 are formedin a direction perpendicular to the rubbing direction 42 (FIG. 4C). Morespecifically, referring to FIGS. 4A-4D, the device 1 includes a sealingmember 12 enclosing a display region 14 and a marginal frame displayregion 15 outside the display region 14, which frame display region 15is driven to always display a black or white state (corresponding to astable +θ or -θ state of the liquid crystal).

When the cell thickness is made sufficiently thin, each liquid crystalmolecule can assume two stable states. Now, it is assumed that all themolecules within the cell are caused to uniformly assume one of the twostable states, which is represented by angle +θ with respect to a normal44 to the smectic layers 41 as shown in FIG. 4D. The other stable stateis represented by an almost symmetrical angle -θ with respect to thesmectic layer normal 44. When the liquid crystal molecules in the stateof +θ are supplied with an electric field (of, e.g., rectangular pulsesof ±8 volts at 10 Hz) applied to the entire area of the device for along period, the liquid crystal molecules begin to move in a directionof from a point A to a point B shown in FIG. 4A. As a result ofcontinuous application of the voltage for a long period, the devicecauses a local change in cell thickness to finally result in parts Evoid of liquid crystal along a side near the point A and the thicknessat the point B becomes larger than the point A.

On the other hand, in case where the liquid crystal molecules areuniformly placed in the state of -θ, the above-mentioned phenomenonappears in such a manner that the liquid crystal molecules movetranslationally from the side B to the side A to results in liquidcrystal voids on the side B.

As a result of further experiments of ours, the above-mentioned cellthickness change due to movement of liquid crystal molecules occurs inremarkably different degrees depending on the molecular state (+θ or -θ)of the liquid crystal relative to the smectic layer normal, thegeometrical length of one molecular state region of the liquid crystalconcerned, and the length of the electric field application time.

This is explained in more detail with reference to FIGS. 5A-5D, whereinFIG. 5A illustrates a state before voltage application and FIG. 5Billustrates a state after voltage application. Referring to FIG. 5A, inan experiment, stripe regions L (where liquid crystal molecules 43 wereuniformly placed in the state of +θ with respect to the smectic layernormal as shown in FIG. 5C) and R (where liquid crystal molecules 43were uniformly placed in the state of -θ as shown in FIG. 5D) wereformed at certain intervals. With lapse of time, the cell thicknessincreased along the side B in the frame display region 15 and, onfurther continuation, liquid crystal voids E occurred only at parts inthe frame display region 15 along the side A. This means that the cellthickness change due to liquid crystal molecular movement occurs in casewhere a region containing one molecular state is continuously present ina large length in the direction of smectic layer extension.

Similar results as shown in FIG. 5B were obtained also in anotherexperiment wherein the liquid crystal molecules only in the displayregion 14 and except for the frame display region 15 were alternatelyplaced in the states of +θ and -θ at certain time intervals. Also fromthis result, it is understood that the cell thickness change occurredonly in case where the liquid crystal molecules were uniformly placed inone state for a long period of time.

The occurrence of such electrooptically uncontrollable portions (e.g.,parts E) is of course undesirable in respects of display quality, andalso the cell thickness change with time leads to a difficulty incontrol of an entire ferroelectric liquid crystal panel. This isparticularly problematic with respect to a durability of a fixed patterndisplay like that of a frame display region driven in only one of the +θand -θ states and also a semi-fixed pattern like a menu screen of a wordprocessor.

SUMMARY OF THE INVENTION

In view of the above-mentioned problems of the prior art device, anobject of the present invention is to provide a chiral smectic liquidcrystal display device with a high durability, which is free fromoccurrence of electro-optically uncontrollable parts, such as liquidcrystal voids, for a long period of time even in a region of a fixedpattern or a semi-fixed pattern display.

According to the present invention, there is provided a liquid crystaldisplay apparatus, comprising: a display panel including a pair ofsubstrates having thereon scanning electrodes and data electrodes so asto form an electrode matrix, and a chiral smectic liquid crystaldisposed between the scanning electrodes and data electrodes to form aplurality of smectic layers each comprising a plurality of liquidcrystal molecules; the scanning electrodes and data electrodes beingdriven to define an effective display region and a frame-like fixed orsemi-fixed pattern display region disposed outside the effective displayregion and extending along at least one side of the display panel, thesmectic layers of the chiral smectic liquid crystal being aligned toextend in a direction which is non-parallel with the extension of theframe-like fixed or semi-fixed pattern display region.

These and other objects, features and advantages of the presentinvention will become more apparent upon a consideration of thefollowing description of the preferred embodiments of the presentinvention taken in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a planar front view of a chiral smectic liquid crystal displaydevice according to an embodiment of the present invention.

FIG. 2 is an illustration of an electrode matrix used in the liquidcrystal display device of FIG. 1.

FIG. 3 is a diagram showing a set of driving voltage waveforms used inan operation example of the device of FIG. 1.

FIGS. 4A-4D are illustrations of liquid crystal molecular movement.

FIGS. 5A-5D are illustrations of liquid crystal molecular movementsimilar to FIGS. 4A-4D.

FIG. 6 is a sectional view of a liquid crystal device for illustratingalignment of chiral smectic liquid crystal molecules.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 is a planar front view of a chiral smectic liquid crystal deviceaccording to an embodiment of the present invention. Referring to FIG.1, the device includes a pair of substrates (glass plates) 11 eachcoated with transparent electrodes of, e.g., In₂ O₃ or ITO (indium tinoxide), and an alignment film, etc., and a ferroelectric chiral smecticliquid crystal disposed between the substrates. The substrates 11 areapplied to each other with a sealing agent or adhesive 12 to form a cellstructure filled with the ferroelectric liquid crystal through aninjection port which is sealed up with a sealing agent 13. Theelectrodes formed on the pair of substrates are arranged to form asimple electrode matrix as shown in a somewhat simplified from in FIG. 2including scanning electrodes S₁ -S₄ in an effective display area anddata electrodes I₁ -I₄ in an effectively display area which incombination define a display region 14 as shown in FIG. 1, and alsoscanning electrodes F_(S1) and F_(S2) for frame display and dataelectrodes F_(I1) and F_(I2) for frame display which in combinationdefine a frame display region 15 as shown in FIG. 1. The scanningelectrodes and data electrodes on the substrates are respectivelyconnected to an electric circuit-printed board carrying a scanning sidedriver IC 16 and a data side driver IC 17. As shown in FIG. 6, thechiral smectic liquid crystal disposed between the pair of substrates61a and 61b respectively provided with rubbing axes 62a and 62b insubstantially identical directions commonly represented by a direction18 in FIG. 1, is aligned to form a plurality of smectic liquid crystalmolecular layers 63 each constituted by a plurality of liquid crystalmolecules 64. The liquid crystal molecular layers (64 in FIG. 6) areformed to extend in a direction 19 perpendicular to the rubbing axis ordirection 18 as shown in FIG. 1. As described, the rubbing axes areprovided in substantially identical directions but can cross each otherso as to form a intersection angle of about 2 degrees to about 15degrees. In the present invention, the liquid crystal molecular layersare formed in the direction 19 which forms a certain angle α withrespect to an extension of a frame display region 15 extending along aside of the panel.

More specifically, in the present invention, the substrates each havingthereon electrodes and an alignment film thereon are subjected to analigning treatment so that the resultant smectic layers do not extend ina direction parallel to a longitudinal direction of a fixed or semifixedpattern which is always or frequently displayed. The aligning treatmentmay be rubbing or oblique vapor deposition. The aligning treatment maypreferably be effected so that the resultant pretilt angles δ, i.e., theangles formed by liquid crystal molecules in the vicinity of thesubstrates with respect to the associated substrates 61a and 61b, areformed in substantially identical directions as shown in FIG. 6. Theliquid crystal may be in a uniform alignment state.

In case where the fixed or semi-fixed pattern display region has alength a and a width b as shown in FIG. 1, the direction of smecticlayer extension may preferably be deviated at an angle α satisfying tanα≧b/a.

Thus, according to the present invention, an aligning treatment iseffected so that a smectic layer passing one longitudinal end of a fixedor semifixed pattern display region is not allowed to extend to passalso the other longitudinal end of the fixed or semifixed patterndisplay region, thereby improving the durability of the liquid crystaldevice. For example, if the smectic layer is disposed in a directionforming an angle of at least 3 degrees, preferably 5-50 degrees, withrespect to the longitudinal extension of a fixed or semifixed patterndisplay region, a practically sufficient durability may be obtained.

Referring to FIG. 2 showing a simple electrode matrix in a somewhatsimplified form, scanning electrodes F_(S1) and F_(S2) for frame displayare disposed outside scanning electrodes, S₁, S₂, S₃ and S₄ for definingan effective display area. The scanning electrodes S₁ -S₄ are suppliedwith voltage waveforms (scanning signals) as shown at S₁, S₂, S₃ . . .in FIG. 3 including a scanning selection signal (having a length of 2H(=4Δt)) and a scanning nonselection signal. The scanning electrodesF_(S1) and F_(S2) are also supplied with a scanning selection signalsimilar to those shown at S₁, S₂ . . . in FIG. 3, at a frequency of,e.g., once per 60 times of supplying a scanning selection signal to thescanning electrodes S₁ -S₄ (actually 1042 lines in a specific example).On the other hand, data electrodes I₁ -I₄ (actually 1124 lines in aspecific example) constituting the reflective display region may besupplied with data signals representatively shown at I₁, I₂, . . . inFIG. 3 corresponding to a desired display pattern. In synchronism withthe application of the scanning selection signal to the scanningelectrode F_(S1) or F_(S2), data signals giving either a "white" or"black" display state are simultaneously supplied. Further, to the dataelectrodes F_(I1) and F_(I2) for frame display disposed outside the dataelectrodes I₁, . . . I₄, data signals giving either a "white" or "black"display state ("black" state in the case of FIG. 3) when synchronizedwith the scanning selection signal are continually supplied.

As a result, a fixed or semifixed display pattern for frame display (ina "black" state in the case of FIG. 3) may be formed.

Hereinbelow, specific examples of durability evaluation will bedescribed.

EXAMPLE 1

A ferroelectric liquid crystal display device (panel) as explained withreference to FIG. 1 was prepared while applying a rubbing treatment tothe substrates so that the smectic layer extended at an angle α of 0.5degree with respect to the longitudinal direction of the frame displayregion. The device was then subjected to a durability test wherein a setof driving signals as described above with reference to FIG. 3 (|V₁|=|V₂ |=10 volts, |V₃ |=|V₄ |=5 volts, 1H=300 μsec) were applied to therespective electrodes (1042 scanning electrodes and 1124 dataelectrodes) continuously to observe the occurrence of a change in cellthickness or liquid crystal voids along the panel.

The liquid crystal material used was an ester-pyrimidine type mixtureliquid crystal showing the following phase transition temperature (°C.):##STR1## The liquid crystal also showed a spontaneous polarization of5.8 nC/cm² at 25° C., and the resultant liquid crystal panel showed aresponse speed (causing a transmittance change of 10-90% at 30 volts) of110 μsec.

As a result, no change in cell thickness or no occurrence of liquidcrystal void was observed over the extension of the panel in thedurability test of 1000 hours. Thus, the panel was evaluated as adisplay device of practically no problem.

EXAMPLES 2-6

Five liquid crystal devices (panels) were prepared and evaluated in thesame manner as in Example 1 except that the angle α was changed to 3degrees, 10 degrees, 20 degrees, 30 degrees and 45 degrees,respectively. The results are shown in Table 1 appearing hereinaftertogether with Example 1.

In any case, results of practically no problem were obtained.

Comparative Example 1

A liquid crystal device was prepared and evaluated in the same manner asin Example 1 except that the angle α shown in FIG. 1 was changed to 0degree, i.e., the rubbing was applied to the substrates so that thesmectic layer extension direction coincided with the longitudinaldirection of the frame display region.

As a result, a cell thickness change (≧0.1 μm) and liquid crystal voidsoccurred in the frame region as shown in FIG. 5 within 500 hours fromthe start of the durability test. The results are shown in Table 1together with the results of Examples 1-6.

                  TABLE 1    ______________________________________           α  Cell thickness                                Liquid crystal           (degrees)                    change      void    ______________________________________    Example 1              5         None        Not occurred    Example 2              3         None        within 1000 hours    Example 3             10         None    Example 4             20         None    Example 5             30         None    Example 6             45         None    Comp.    Example 1              0         Occurred after                                    Occurred after                        280 hours in                                    360 hours in                        frame region                                    frame region    ______________________________________

As is shown in the above Table 1, it is known that the durability of aferroelectric liquid crystal device can be improved to a level ofpractically no problem if the angle α is set to at least 3 degrees,preferably 5 degrees-50 degrees.

As has been described above, according to the present invention, thedurability of a ferroelectric liquid crystal device can be improved byapplying an aligning treatment so that the resultant smectic layerextension is not in parallel with the longitudinal direction of a fixedor semifixed display pattern region.

What is claimed is:
 1. A liquid crystal display apparatus, comprising:adisplay panel including a pair of substrates having thereon scanningelectrodes and data electrodes so as to form an electrode matrix, achiral smectic liquid crystal disposed between the scanning electrodesand data electrodes to form a plurality of smectic layers eachcomprising a plurality of liquid crystal molecules, said smectic layersbeing inclined with respect to both substrates and bent between thesubstrates, and drive means for applying (i) a scanning signal to thescanning electrodes in an effective display region in synchronism withdata signals to the data electrodes in the effective display region and(ii) in a prescribed period a scanning-side drive pulse having awaveform identical to that of the scanning signal to the scanningelectrodes outside the effective display region in synchronism withdata-side drive pulses for providing a frame-like fixed or semi-fixedpattern to the data electrodes outside the effective display region;said scanning signal including a scanning selection signal and ascanning non-selection signal, said scanning selection signal comprisinga first voltage of one polarity and a second voltage of the otherpolarity immediately subsequent to the first voltage, the first voltagesof two successive scanning selection signals partially overlapping witheach other in time; wherein said pattern display region extends along atleast one side of the display panel, said smectic layers of the chiralsmectic liquid crystal being aligned to extend in a direction which isnon-parallel with the extension of the pattern display region.
 2. Anapparatus according to claim 1, wherein said smectic layers are alignedto extend in a direction at an angle α of at least 3 degrees withrespect to the extension of the frame-like fixed or semi-fixed patterndisplay region.
 3. An apparatus according to claim 2, wherein the angleα is set within the range of 5 degrees to 50 degrees.
 4. An apparatusaccording to claim 1, wherein said liquid crystal molecules within thesmectic layers form a pre-tilt angle in the vicinity of the substrates.5. An apparatus according to claim 1, wherein a scanning electrodedriven to form the fixed or semi-fixed pattern display region issupplied with a scanning signal which is identical in waveform to ascanning selection signal applied to the scanning electrodes fordefining the effective display region.
 6. An apparatus according toclaim 1, wherein a data electrode driven to form the fixed or semi-fixedpattern display region is continually supplied with a display signalproviding one display state to pixels associated with said dataelectrode.
 7. An apparatus according to claim 1, wherein said substrateshave been treated by rubbing for aligning the chiral smectic liquidcrystal.
 8. An apparatus according to claim 1, wherein said substrateshave been treated by oblique vapor deposition for aligning the chiralsmectic liquid crystal.
 9. An apparatus according to claim 1, whereinsaid scanning selection signal further comprises a voltage of said onepolarity after said voltage of the other polarity.
 10. An apparatusaccording to claim 1, wherein the voltage of said other polarity of thefirst of said two successive scanning selection signals overlaps in timewith the voltage of said one polarity of the second of said twosuccessive scanning selection signals.
 11. An apparatus according toclaim 1, wherein said scanning non-selection signal has a voltage ofzero.
 12. An apparatus according to claim 1, wherein said chiral smecticliquid crystal has a property of moving in a direction of its smecticlayer under application of an electric field.